A disk drive is a digital data storage device that stores information on concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the magnetic transducer while the transducer is positioned above (or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the magnetic transducer that induces magnetically polarized transitions on the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the magnetic transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.
When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above a desired track. In an on-rack mode, the transducer reads data from or writes data to the desired track. The tracks are typically not completely circular. Accordingly, in the on-track mode the transducer must be moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.
Modern hard disk drives employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances. The present invention relates to hard disk drives having single or dual-stage actuator systems.
FIG. 1 depicts a disk drive 10 comprising an electronic portion 10a and a mechanical portion 10b. The electronic portion 10a comprises control electronics typically including a preamplifier, a read/write channel, a servo control unit, a random access memory (RAM), and read only memory (ROM), spindle motor and VCM controller driving electronics. The electronic portion 10a is or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.
FIGS. 2 and 3 show that the mechanical portion 10b of the disk drive 10 includes a disk 12 that is rotated by a spin motor 14. The spin motor 14: is mounted to a base plate 16. The disk drive 10 includes at least one and typically a plurality of disks 12, each with one or two recording surfaces.
During use, the disk 12 is rotated about a spindle axis A. The disk drive 10 further comprises what is commonly referred to as a head 18. The head 18 comprises or supports the magnetic read/write transducer described above and will thus be referred to herein as the component of the disk drive 10 that reads data from and writes data to the disk 12.
FIGS. 2 and 3 further illustrate a positioning system 20 of the disk drive 10. The positioning system 20 comprises a bearing assembly 22 that supports at least one actuator arm assembly 24. The actuator arm assembly 24 supports the head 18 adjacent to one recording surface 26 of one of the disks 12. Typically, the bearing assembly 22 will support one actuator arm assembly 24 and associated head 18 adjacent to each of the recording surfaces 26 of each of the disks 12. The actuator arm assemblies 24 allow each head 18 to be moved as necessary to seek to a desired track 34 in seek mode and then follow the desired track 34 in track following mode.
The exemplary positioning system 20 depicted in FIGS. 2 and 3 is a dual-stage system. Accordingly, each actuator arm assembly 24 comprises a first actuator 30 and a second actuator 32. The principles of the present invention are currently of primary importance when applied to the second actuator of a dual-stage actuator system, and that application of the present invention will be described herein. However, the present invention may in the future have application to a single stage actuator system. The scope of the present invention should thus be determined by the claims appended hereto and not the following detailed discussion.
For ease of illustration, FIGS. 2 and 3 depict the first and second actuators 30 and 32 as comprising elongate arms 36 and 38, respectively, and the actuators 30 and 32 may be implemented as shown in FIGS. 2 and 3. Conventionally, the bearing assembly 22 is also considered part of the first actuator 30. In particular, the bearing assembly 22 supports a proximal end 40 of the arm 36 of the first actuator 30 for rotation about a first axis B, while a distal end 42 of the first actuator arm 36 supports a proximal end 44 of an arm 38 of the second actuator 32 for rotation about a second axis C. In this case, the head 18 is supported on a distal end 46 of the second actuator arm 38.
The actuators 30 and 32 may, however, be implemented using other structures or combinations of structures. For example, the first actuator 30 may comprise an elongate arm that rotates about a first axis B, while the second actuator 32 may comprise a suspension assembly rigidly connected to a distal end of the first actuator. In this case, the first actuator is able to rotate about an actuator axis, while the head 18 would be suspended from the second actuator for linear movement along the disk radius relative to the position of the first actuator. The actuators 30 and 32 may thus take any number of physical forms, and the scope of the present invention should not be limited to the exemplary actuators 30 and 32 depicted in FIGS. 2 and 3.
FIG. 3 also illustrates that the exemplary actuators 30 and 32 of the positioning system 20 further comprise a first electromechanical transducer 50 and a second electromechanical transducer 52. In response to a first control signal, the first transducer 50 moves the first actuator arm 36 to change an angular position of the head 18 relative to the first axis B. The second transducer 52 is supported by the distal end 42 of the first actuator 30 to rotate the head 18 about the second axis C in response to a second control signal. In FIG. 3, an angular position of the first actuator arm 36 is represented by reference character D, while an angular position of the second actuator arm 38 is represented by reference character E.
A range of movement “S” associated with the second transducer 32 is defined by the stroke “s+” and “s−” in either direction relative to a neutral position D defined by the first actuator arm 36. The term “actual displacement” (ds in FIG. 3) refers to the angular difference at any point in time of the head 18 relative to the neutral position as defined by the position D of the first actuator 30. When the head 18 is in the neutral position, the actual displacement of the second actuator arm 38 is zero.
FIG. 3 further identifies arbitrary first and second tracks 34a and 34b on the disk 12. The actuator arm assembly 24 is shown in an initial position by solid lines and in a target position by broken lines; the first track 34a will thus be referred to as the “initial track” and the second track 34b will be referred to as the “target track”. It should be understood that the terms “initial track” and “target track” are relative to the position of the head 18 before and after a seek operation. Any track 34 on the disk 12 may be considered the initial track or the target track depending upon the state of the disk drive 10; before and after a particular seek operation.
The present invention is of particular importance in the context of an electromechanical transducer for a hard disk drive comprising two or more displacement elements operated by a single control signal. In particular, the secondary actuator 32 of the exemplary disk drive 10 can be implemented using displacement elements such as microelectromechanical system (MEMS) transducers or piezoelectric transducers. The present invention is also of particular significance when the displacement elements comprise piezoelectric material. The present invention will be thus described herein in the context of a dual-stage actuator for a hard disk drive in which the second stage is formed by two or more piezoelectric elements.
It should be noted, however, that the principles of the present invention can also be applied to an electromechanical transducer of a single stage actuator system or to an electromechanical transducer used to displace the first actuator of a dual-stage actuator system.
Piezoelectric materials are materials that mechanically deform when an electric field is applied thereto. A single piezoelectric element can be used as a piezoelectric actuator. An electromechanical transducer for the second stage actuator of a dual-stage actuator system of a disk drive can be implemented as a pair of piezoelectric transducer elements driven in opposite directions by a single control signal.
Commercially available piezoelectric materials are typically ferroelectric ceramics containing crystal dipoles that, initially, are randomly oriented. During what is referred to as the poling process, an electric field is applied to the ceramic material to cause the dipoles to become aligned. After the electric field is removed, the dipoles remain in substantial alignment and the material exhibits piezoelectric properties. A piezoelectric ceramic can, however, become depoled if an electric field is applied to the material in a direction opposite to that of the original poling electric field. Accordingly, a bias voltage is typically applied to piezoelectric elements during use to ensure that the piezoelectric material is always subjected to an electric field in the same direction as the original poling field.
FIG. 4 depicts a block diagram of an example of a prior art displacement system 60 that may be used as the second transducer 52. The displacement system 60 comprises first and second piezoelectric displacement elements 62 and 64 and a driving circuit 66. The mechanical coupling between the first and second displacement elements 62 and 64 and the head 18 depends upon the nature of the second actuator 32 and head 18. Also, as described above, the second transducer 52 may be implemented using alternative technologies such as MEMS actuators, which may also change the nature of the mechanical coupling between the head 18 and the displacement elements 62 and 64.
The driving circuit 66 comprises a signal source 70 and first and second amplifier circuits 72 and 74. The signal source 70 generates a control signal v(t) from an error signal representative of a difference between the position of the head 18 and the position of the desired track 34. As described in the following equations (1) and (2), the amplifier circuits 72 and 74 generate respective first and second driving signals vA(t) and vB(t) from the control signal v(t) and a no signal bias level Vbias, where:vA(t)=Vbias+v(t)  (1) vB(t)=Vbias−v(t)  (2) 
The driving signals vA(t) and vB(t) are applied directly across the displacement elements 62 and 64 to cause these elements 62 and 64 to deform and thereby displace the transducer head 18 relative to the first actuator 30 as determined by the control signal v(t).
The no signal bias level Vbias of the driving signals vA(t) and vB(t) is predetermined so that the instantaneous range of values for vA(t) and vB(t) lies between the reference and supply voltages as shown in FIG. 5. Also as shown in the example depicted in FIG. 5, the resulting driving signals vA(t) and vB(t) are the inverse of each other and vary within an operational range above zero volts and less than VSOURCE. The waveforms depicted in FIG. 5 are for illustration purposes only and are not representative of the actual control signal v(t).
When applied directly across the first and second displacement elements 62 and 64, the first and second driving signals vA(t) and vB(t) cause the displacement elements 62 and 64 to move in opposition to each other.
In addition, the driving signals vA(t) and vB(t) are always positive relative to the polarity of the displacement elements 62 and 64. The driving signals vA(t) and vB(t) thus ensure that the piezoelectric material forming the elements 62 and 64 are always subject to a positive electric field and are thus not depoled during normal operation.
A need exists, however, for a drive system for the secondary actuator system of a dual-stage actuator of a disk drive that is less complex and less expensive to implement than conventional secondary actuator systems.